A cellulose acylate film is used in various photographic or optical elements because it is tough and has enough flame retardant properties. In fact, the cellulose acylate film is a representative photographic support. Further, having optical isotropy, the film is also used in a liquid crystal display device, which has recently extended its market. In the device, the cellulose acylate film often serves as a protective film of a polarizing plate or a color filter.
A cellulose acylate film is generally produced according to a solvent cast method or a melt cast method. The solvent cast method comprises the steps of casting a solution of cellulose acylate dissolved in a solvent (that is called “dope”) onto a support, and evaporating the solvent to form a film. On the other hand, the melt cast method comprises the steps of heating cellulose acylate to melt, casting the melt on a support, and cooling to form a film. Practically, the solvent cast method is generally adopted because it can form a highly flat film, as compared with the melt cast method. The solvent cast method is described in various documents. In some recently proposed processes of the solvent cast method, it is aimed to shorten the time between the steps of casting the dope on the support and peeling the formed film off, so that the productivity of the film formation may be improved. For examples, Japanese Patent Publication No. 5(1993)-17844 discloses a process in which a concentrated dope is cast onto a cooling drum to shorten the time between the casting step and the peeling step.
The solvent used in the solvent cast method must have functions of not only dissolving cellulose acylate but also forming an excellent film. In more detail, the viscosity and the polymer concentration of the solution (dope) should be appropriately adjusted to form a flat film having a uniform thickness. The dope also should be easily set to gel. Further, the formed film should be easily peeled from the support. In order to satisfy these requirements, the most appropriate solvent should be selected. Moreover, the solvent should be so easily evaporated that it scarcely remains in the film.
Various organic solvents have been proposed as the solvent of cellulose acylate, but only methylene chloride has been practically used. However, since methylene chloride has defects in view of environment protection and the boiling point, a new solvent substituting for methylene chloride has been searched.
J. M. G. Cowie et al. report in Makromol., Chem., 143 (1971) 105, that cellulose acylate having a substitution degree in the range of 2.80 to 2.90 is dissolved in acetone through a specific process. The process comprises the steps of cooling the cellulose acylate (in the reported process the acyl group is restricted to acetyl group) in acetone at a temperature of −80 to −70° C., and warming it to obtain 0.5 to 5 wt. % solution of cellulose acylate in acetone. The method, in which the mixture of cellulose acylate is cooled in an organic solvent to obtain a solution, is hereinafter referred to as “cooling dissolution method”. The solution of cellulose acylate in acetone is also reported by K. Kamide et al., Bulletin of Textile Machinery Society, Vol. 34, pp. 57 (1981). The report (written in Japanese) is entitled “Dry spinning process using acetone solution of triacetyl cellulose”. In the report, the cooling dissolution method is applied to the art of fiber spinning. The experiments described in the report examine the mechanical strength, the dyeing property and the sectioned shape of the fiber obtained by the cooling dissolution method. In the report, 10 to 25 wt. % solution of cellulose acetate is used to form a fiber.
Methods other than the cooling dissolution method can be adopted to prepare the cellulose acylate solution. For example, “high temperature method”, in which cellulose acylate is dissolved at high temperature and high pressure, is proposed.
A cellulose acylate film is produced by the solvent cast method, which comprises the steps of casting the above-described solution of cellulose acylate (dope) onto a support, evaporating the solvent to form a film, and peeling the film from the support. The thus-formed film, however, often has an undesirable surface. In detail, the dope is often so viscous that striped traces are formed in casting and not leveled to remain on the film surface. As a result, the formed film does not have a smooth surface and the striped traces are sometimes severe enough to be recognized as striped defects. In order to obtain a good surface, the dope must be diluted or the cast dope must be dried under moderate conditions. However, these measures increase the production cost and impair the production speed, and hence it has been wanted to develop new measures.